As is known to those of ordinary skill in the art, there is a preferred ratio between the diameter of the high-pressure turbine spool (including the high-pressure turbine nozzle and the associated high-pressure turbine rotor) and the diameter of the associated low-pressure turbine spool (including the low-pressure turbine nozzle and the associated low-pressure turbine rotor) in a gas turbine engine, with the low-pressure turbine spool having a relatively larger diameter than the high-pressure turbine spool. As described herein below, this ratio provides optimal gas turbine engine performance. However, in order to achieve this preferred ratio, the slope of the outer-body surface of the inter-turbine duct joining the high-pressure turbine spool and the low-pressure turbine spool must be relatively steep for a given inter-turbine duct length. Alternatively, the length of the outer-body surface of the inter-turbine duct may be relatively long. Given conventional systems, assemblies, and methods, either configuration is problematic.
If the slope of the outer-body surface of the inter-turbine duct is relatively steep, strong adverse pressure gradients may be created along the outer-body surface and boundary layer separation may occur in the inter-turbine duct. As a result, gas turbine engine performance may suffer. If the length of the outer-body surface of the inter-turbine duct is relatively long, extra weight is added to the gas turbine engine and, again, performance may suffer. Thus, conventional gas turbine engines typically use non-optimal inter-turbine ducts that are relatively long and that have outer-body surfaces with a relatively shallow slope. For example, an inter-turbine duct with a length of approximately twice the height of the high-pressure turbine rotor and a slope of about 31 degrees may be used.
Thus, what is needed are systems, assemblies, and methods that minimize the creation of adverse pressure gradients along the outer-body surface and prevent boundary layer separation in the inter-turbine duct, allowing for the manufacture and use of an outer-body surface that has a relatively steep slope. This would allow either a relatively short inter-turbine duct to be used, minimizing weight, or a relatively higher diameter low-pressure turbine that optimizes gas turbine engine performance. What is also needed are systems, assemblies, and methods that provide such an inter-turbine duct relatively inexpensively and using conventional manufacturing techniques and materials.